1. Field of the Invention
The present invention relates to stacked or coiled type metallized flexible dielectric film capacitors as well as to a method for making these capacitors.
2. Description of the Prior Art
The fabrication of metallized, flexible dielectric film capacitors is well known to those skilled in the art. As shown in FIG. 1, which pertains to the main steps in the fabrication of a stacked type of metallized, flexible dielectric film capacitor, this fabrication can be divided into several main steps. Step 1 consists in the metallization of a substantially wide flexible, dielectric film which may be a polyester, polycarbonate, polysulfone or polypropylene film. The metallization is done by a process of evaporation under vacuum. At present, this metallization of the film is done with non-metallized margins that are obtained during the process of evaporation under vacuum by providing for masking bands as represented in FIG. 1 by the reference C.
In a second step, the substantially wide metallized film is cut out into rolls of film of a desired width. To be capable of being used in the fabrication of capacitors, these rolls of film should be constituted by even or odd rolls of film. To obtain these even or odd rolls of film, as shown in the step 2 of FIG. 1, the substantially wide metallized film should be cut out in the middle of the margins m and in the middle of the metallized part m' between two margins, as shown respectively by the references A and A' in FIG. 1.
During the step 3, the two films, f, f', which are the even film and the odd film respectively, are wound on a large diameter wheel referenced R. As shown by the reference S, the winding of the even and odd films is done by creating an offset between the two films called a projection or projecting part d. This winding on a large-diameter wheel enables a parent capacitor to be obtained. Then, in a known manner, during the step 4, a metallization is sprayed on each side of the parent capacitor thus made so as to set up the lateral electrical connections. The metallization is preferably done by Schoop's metal spraying process. As shown in FIG. 1, this Schoop's metal spraying process enables the connection, on one side, of the even films referenced f and, on the other side, of the odd films referenced f'.
Once all these operations are over, therefore, a parent capacitor is obtained. The parent capacitor is then cut out so that individual capacitors are obtained, as shown in FIGS. 2A and 2B. This capacitor is generally parallelepiped shaped. It is formed by a stacking of alternating even-order and odd-order layers referenced 1 and 2. Each layer is formed by a dielectric-forming flexible plastic film. On one of its faces, each dielectric film has a metallized zone 7 and a non-metallized lateral margin 3 with a width m. Furthermore, as shown in FIGS. 2, electrical connections have been deposited on each of the lateral faces 4, 5 of the capacitor. These lateral connections enable the electrical and mechanical connection of the metallized zones of all the layers of the same order by respectively connecting the parts 6 and the parts 8 of the metallized zones. In this case, therefore, we obtain an active width a for the capacitor. This width corresponds to the facing part between two successive metallized zones. This width a is bordered, on each side, by a width m corresponding to the lateral margin of each film, namely the non-metallized part. This margin is necessary in the fabrication method used in order to avoid short-circuits between the even and odd layers that might be connected during the Schoop metal spraying process or the electric arc at the instant of the charging or discharging, said arc generating a high current that favors the peak effects of the thin metallized edge. Furthermore, on the capacitor, there is a distance d, on each side, called a projecting portion or projection. This distance d results from the fact that the winding of the metallized films is done with two films that have a slight lateral offset with respect to each other. In the methods used at present, the margin m has a width of about 4/10th while the projection d has a width of 1/10th to 2/10th.
This lateral margin, which is necessary for the making of the capacitors according to the method described with reference to FIG. 1, is by nature a width on a non-active film. This therefore means that there is a volume, on the component, that cannot be used by the user. Furthermore, the method implemented to make these capacitors has a number of drawbacks. For, it is necessary to make margins that may have very small widths. This calls for techniques of ever greater sophistication. In addition, the centering of the margins during the metallization of the large-width dielectric film is a delicate operation. The masks used to make the margins are increasingly brittle and costly: this is notably so for the masks used for margins of small width. Furthermore, when the substantially wide metallized film is being cut out, the centering of the knives in the metallized part and in the non-metallized part should be done with high precision. Moreover, it is necessary to identify and manage the film rolls in order to know what they are intended to be i.e. to know whether they are even film rolls or odd film rolls.
The present invention is therefore aimed at overcoming these drawbacks by proposing the making of stacked or coiled type metallized flexible dielectric film capacitors that no longer use metallized films with margins.